1. Field of the Invention
The present invention relates to a method of forming an insulating film on a grain-oriented silicon steel sheet. Particularly, the present invention relates to a method of forming a highly adhesive and uniform insulating film on a grain-oriented silicon steel sheet having a low iron loss.
2. Description of the Prior Art
Grain-oriented silicon steel sheet is generally produced through a series of steps, wherein a silicon steel raw material containing not more than 4.0% by weight of silicon is hot rolled, the hot rolled sheet is annealed and then subjected to one cold rolling or two cold rollings with an intermediate annealing between them to produce a cold rolled sheet having a final gauge, and the cold rolled sheet is subjected to a primary recrystallization annealing to remove carbon in the steel sheet at the same time and then subjected to a final annealing to develop secondarily recrystallized grains having a (110)[001] orientation and at the same time to remove harmful impurities and to form a forsterite insulating film.
The forsterite film formed at the final annealing in the above described conventional method is not present uniformly and smoothly only on the surface layer of the steel sheet. That is, in the film formed by the conventional method, as illustrated by a photomicrograph (magnification: 1,000) of the cross-section of the film formed on the surface of the steel sheet shown in FIG. 1, a large number of forsterite grains are present not only on the outermost surface layer of the steel sheet, but also even at a position several .mu.m directly beneath the surface of the steel sheet. The forsterite grains formed directly beneath the surface of a silicon steel sheet hinder the magnetic domain wall migration and cause a high iron loss of the steel sheet.
In the conventional method, in order to decrease the formation of those forsterite grains, the decarburizationannealing is carried out so as to form a thin subscale layer, whereby a forsterite film having a thickness of as small as possible is formed at the final annealing. Although the forsterite film formed by the above described method is satisfactory in the magnetic properties, the film is extremely poor in the adhesion and further is insufficient in the electric insulating property. That is, when it is intended to secure practically satisfactory film properties by the conventional method, forsterite grains are always formed directly beneath the steel sheet surface.